1. Field of the Invention
The present invention relates to a diamond base material which may suitably be used for fabricating a surface acoustic wave device (hereinafter, mainly referred to as "SAW device"), and a SAW device utilizing such a diamond base material.
2. Related Background Art
Diamond has a highest acoustic propagation velocity among all of the materials which are present on the earth, and has a band gap energy of 5.5 eV, which is very large as-compared with those of known semiconductor substances. In addition, diamond is transparent with respect to light having a wavelength ranging from a near-ultraviolet region to a near-infrared region. Accordingly, it is expected that diamond can improve an operating characteristic and/or can widen an operating range or latitude in various technical or scientific fields such as acoustic, surface acoustic wave, optical, and semiconductor fields.
In addition, since diamond has a good thermal conductivity, not only diamond itself may desirably function as a heat sink material, but also a stable functional device or element (such as optical functional device) which requires substantially no temperature compensation for heat may be constituted by use of diamond.
By utilizing such dynamical, electric or electronic characteristics, diamond has been used for a wide variety of dynamical, optical, electric or electronic devices (as described in, e.g., "Diamond Thin Film", written by Tadao Inuzuka, pp. 99-115, 1990, published by Kyoritsu Shuppan (Tokyo, JAPAN)). Specific examples of such a device utilizing diamond may include a SAW device which may suitably be used for a high-frequency filter, etc.
Heretofore, as the above-mentioned SAW device, there is known one having a multilayered structure comprising a diamond thin film and a combination of an interdigital transducer or electrode having a comb-like shape (hereinafter, mainly referred to as "IDT") and a piezoelectric substance disposed on the diamond thin film, as disclosed in Japanese Patent Publication (KOKOKU) No. 38874/1979 (i.e., Sho 54-38874) and Japanese Laid-Open Patent Application (KOKAI) No. 62911/1989 (i.e., Sho 64-62911).
As described above, the SAW device has a structure comprising a diamond thin film and a piezoelectric layer disposed thereon, wherein the piezoelectric layer comprises a piezoelectric substance such as ZnO, quartz crystal, LiNbO.sub.3 and LiTaO.sub.3. Accordingly, the above-mentioned IDT constituting the SAW device is usually grounded by using a wire bonding method, i.e., a method wherein the IDT is grounded by use of a wire which is electrically connected to the IDT by way of a bonding pad.
However, the above-mentioned conventional wire bonding method has the following drawbacks.
(1) In the wire bonding method, it is necessary to provide, on a device to be wire-bonded, a sufficient space corresponding to the area of bonding pads to be used for the wire bonding, and therefore the size of the device itself necessarily becomes large to a considerable extent.
(2) In a case where the SAW device has a multi-electrode structure comprising a plurality of electrodes (inclusive of IDTs), the number of the bonding wires and the number of the bonding pads become considerably large, and therefore the size of the device itself becomes large to a further extent. In addition, the location or position of the bonding pads is considerably limited so that the bonding wires may be connected to the resultant bonding pads.
(3) In a case where the SAW device has a multi-electrode structure, the bonding steps themselves for conducting the wire bonding become complicated and troublesome.
Particularly, in recent years, there has strongly been demanded a small-sized SAW device of a surface-mounting type, i.e., a SAW device which may be mounted to a surface or surface portion of an apparatus wherein the SAW device is to be utilized. Accordingly, there has been desired the development of a grounding method which is suitably applicable to such a small-sized SAW device of the surface-mounting type.